Project #4. Hemophilia B is an X-linked bleeding disorder resulting from a deficiency of coagulation factor IX. Studies of patients treated with prophylactic factor IX protein infusions to maintain plasma levels >1% show that the chronic arthropathy and life-threatening hemorrhages associated with the disease may be prevented. This experience and the ongoing concern about blood-borne diseases transmitted by the use of factor concentrates forms the rationale for a gene transfer approach to treating hemophilia. Our group has developed a solid preclinical experience with gene transfer using an adeno-associated viral (AAV) vector to mediate transfer of the gene for factor IX to muscle. We have shown an absence of local or systemic toxicity due to AAV injection in rodents and dogs, and demonstrated proof of principle that ultrasound- guided intramuscular administration of AAV containing a species-specific transgene in dogs with hemophilia B results in expression of factor IX in muscle and therapeutically meaningful levels of factor IX (1-2%) in the plasma. Persistent of high-titer antibodies to factor IX or the presence of vector sequences in the semen have been demonstrated in this large animal model. Herein, we propose to carry out studies in humans with severe hemophilia B. Aim #1 details 2 studies using AAV to direct expression of human factor IX in muscle in patients with hemophilia B after ultrasound guided injection of vector into muscle. In the first a dose-escalation study 3 groups of 3 patients will be evaluated for toxicity. The second is a dose finding and efficacy study to determine the dose of AAV-hFIX that results in 5-7% plasma factor IX levels (0.25- 0.35 mug/mL), and to show efficacy of this dose in a group of approximately 25 patients by assessing number of bleeds, factor concentrate use and clinical effect using a hemophilia-specific health assessment tool that we will design. In Aim #2 we will measure levels of circulating reporter gene in rabbits following ultrasound guided administration of AAV in order to determine the optimal volume of injectate, concentration of vector, and the number of injection sites. In Aim #3, we will characterize the human immune response to vector proteins and the expressed factor IX transgene, including surveillance for the formation of anti-factor IX. Experiments in ultrasound guided injection of vector. Given the preclinical experience using this strategy, the proposed studies are likely to result in the first long-lived meaningfully correction of a human genetic disorder.